Atticus tells the men to be quiet because Mr. Robinson was sleeping in his jail cell. This is the sickening comic aspect since the men are there to kill Tom so whether or not he's sleeping makes no difference. Also the respect Atticus is asking the men to give Tom by being quiet is comical since they all start talking in whispers.
Their behavior shows that the men respect Atticus and individually are decent people. This shows that Atticus has power over the men and knows how to defuse what could be a very difficult situation. By having the men comply with his request for them to speak quietly, he is more likely to convince them to abandon their goal for the night.
